Taking your advice, I've toasted it and done a frugal install. After tweaking the bootlocal.sh to do the ifconfig and ip_forwardiing-stuff, I got it to save the changes. And now it seems to come up, config the NICs and just sit there, exactly what I want it to do. I'm still testing but it looks very promising.

My testing proved premature. Upon further rumination, one of the CD boot options is "nodhcp" which does exactly that.

So I re-installed (frugal grub) and when the installation procedure prompted me for boot options, I entered "nodhcp" and "ssh" and completed the installation. I then added all the NIC configs, ip_forward and sshd commands into /opt/bootlocal.sh. I added a password for root and added /etc/shadow to the .filetool.lst.

Now she boots unattended, configures the NICs, starts sshd (so I can get into her should the need arise) and start forwarding packet immediately.